DESCRIPTION FEATURES APPLICATIONS BLOCK DIAGRAM

RS6508
2A, 20V, 400KHz DC/DC Asynchronous
Step-Down Converter
DESCRIPTION
The RS6508 is a high-efficiency asynchronous
step-down DC/DC converter that can deliver up to 2A
output current from 4.75V to 20V input supply. The
RS6508's current mode architecture and external
compensation allow the transient response to be
optimized over a wide range of loads and output
capacitors. Cycle-by-cycle current limit provides
protection against shorted outputs and soft-start
eliminates input current surge during start-up.
The RS6508 also provides output under voltage
protection and thermal shutdown protection. The low
current (<30μA) shutdown mode provides output
disconnection, enabling easy power management in
battery-powered systems.
FEATURES
•
•
•
•
•
•
•
•
•
•
•
2A output current
Up to 93% efficiency
Integrated 100mΩ power MOSFET switches
Fixed 400KHz frequency
Cycle-by-Cycle over current protection
Thermal shutdown function
Wide 4.75V to 20V operating input range
Output adjustable from 0.925V to 18V
Programmable under voltage lockout
Available in an SOP-8 package
RoHS compliant and 100% Lead (Pb)-Free and
Green (Halogen Free with Commercial Standard)
APPLICATIONS
•
•
•
•
•
•
•
•
•
•
•
PC motherboard, graphic card
LCD monitor
Set-top boxes
DVD-video player
Telecom equipment
ADSL modem
Printer and other peripheral equipment
Microprocessor core supply
Networking power supply
Pre-regulator for linear regulators
Green electronics/appliances
BLOCK DIAGRAM
Tel: 886-66296288‧Fax: 886-29174598‧ http://www.princeton.com.tw‧2F, No. 233-1, Baociao Rd., Sindian Dist., New Taipei City 23145, Taiwan
RS6508
APPLICATION CIRCUIT
ORDER INFORMATION
Device
RS6508-XX Y Z
V1.0
Device Code
XX is nominal output voltage:
AD: ADJ
Y is package & Pin Assignments designator:
S: SOP-8
Z is Lead Free designator:
P: Commercial Standard, Lead (Pb) Free and Phosphorous (P) Free Package
G: Green (Halogen Free with Commercial Standard)
2
March 2013
RS6508
PIN ASSIGNMENTS
SOP-8
PIN DESCRIPTION
Pin Name
BS
IN
SW
GND
FB
COMP
EN
SS
V1.0
Description
Bootstrap. This capacitor (C5) is needed to drive the power switch’s gate above the
supply voltage. It is connected between the SW and BS pins to form a floating supply
across the power switch driver. The voltage across C5 is about 5V and is supplied by the
internal +5V supply when the SW pin voltage is low.
Supply Voltage. The RS6508 operates from a 4.75V to 20V unregulated input. C1 is
needed to prevent large voltage spikes from appearing at the input.
Power Switching Output. SW is the switching node that supplies power to the output.
Connect the output LC filter from SW to the output load. Note that a capacitor is required
from SW to BS to power the high-side switch.
Ground.
Feedback Input. FB senses the output voltage and regulates it. Drive FB with a
resistive voltage divider from the output voltage to ground. The feedback threshold is
0.925V. See Setting the Output Voltage.
Compensation Node. COMP is used to compensate the regulation control loop.
Connect a series RC network from COMP to GND. In some cases, an additional
capacitor from COMP to GND is required. See Compensation.
Enable Input. EN is a digital input that turns the regulator on or off. Drive EN high to turn
on the regulator, drive it low to turn it off. For automatic startup, leave EN unconnected.
Soft-Start. Connect SS to an external capacitor to program the soft-start. If unused, leave
it open.
3
Pin No.
1
2
3
4
5
6
7
8
March 2013
RS6508
FUNCTION DESCRIPTION
The RS6508 is a synchronous high voltage buck converter that can support the input voltage range from 4.75V to 20V
and the output current can be up to 2A.
OUTPUT VOLTAGE SETTING
The resistive divider allows the FB pin to sense the output voltage as shown in Figure 1.
Figure 1. Output Voltage Setting
The output voltage is set by an external resistive divider according to the following equation:
R1 

VOUT = VFB 1 +

 R2 
Where VFB is the feedback reference voltage (0.925V typ.).
EXTERNAL BOOTSTRAP DIODE
Connect a 10nF low ESR ceramic capacitor between the BOOT pin and SW pin. This capacitor provides the gate driver
voltage for the high side MOSFET.
It is recommended to add an external bootstrap diode between an external 5V and the BOOT pin for efficiency
improvement when input voltage is lower than 5.5V or duty ratio is higher than 65%. The bootstrap diode can be a low
cost one such as 1N4148 or BAT54.
SOFT-START
The RS6508 contains an external soft‐ start clamp that gradually raises the output voltage. The soft-start timing can be
programmed by the external capacitor between SS pin and GND. The chip provides a 7μA charge current for the
external capacitor. If a 0.1μF capacitor is used to set the soft-start and its period will be 13ms(typ.).
INDUCTOR SELECTION
The inductor value and operating frequency determine the ripple current according to a specific input and output voltage.
The ripple current ΔIL increases with higher VIN and decreases with higher inductance.

V
  V
ΔI L =  OUT  × 1 − OUT 
V IN 
 f ×L  
Having a lower ripple current reduces not only the ESR losses in the output capacitors but also the output voltage ripple.
High frequency with small ripple current can achieve highest efficiency operation. However, it requires a large inductor to
achieve this goal.
V1.0
4
March 2013
RS6508
For the ripple current selection, the value of ΔIL=0.2375(IMAX) will be a reasonable starting point. The largest ripple
current occurs at the highest VIN. To guarantee that the ripple current stays below the specified maximum, the inductor
value should be chosen according to the following equation:
 VOUT
 
VOUT 
L=
 × 1 −

 f × ΔIL(MAX )   V IN (MAX ) 
INDUCTOR CORE SELECTION
The inductor type must be selected once the value for L is known. Generally speaking, high efficiency converters can not
afford the core loss found in low cost powdered iron cores. So, the more expensive ferrite or mollypermalloy cores will be
a better choice.
The selected inductance rather than the core size for a fixed inductor value is the key for actual core loss. As the
inductance increases, core losses decrease. Unfortunately, increase of the inductance requires more turns of wire and
therefore the copper losses will increase.
Ferrite designs are preferred at high switching frequency due to the characteristics of very low core losses. So, design
goals can focus on the reduction of copper loss and the saturation prevention.
Ferrite core material saturates “hard”, which means that inductance collapses abruptly when the peak design current is
exceeded. The previous situation results in an abrupt increase in inductor ripple current and consequent output voltage
ripple.
Do not allow the core to saturate! Different core materials and shapes will change the size/ current and price/current
relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy materials are small and do not radiate energy. However, they are
usually more expensive than the similar powdered iron inductors. The rule for inductor choice mainly depends on the
price vs. size requirement and any radiated field/EMI requirements.
CIN AND COUT SELECTION
The input capacitance, CIN, is needed to filter the trapezoidal current at the source of the high side MOSFET. To prevent
large ripple current, a low ESR input capacitor sized for the maximum RMS current should be used. The RMS current is
given by:
V
VIN
−1
IRMS = IOUT (MAX ) × OUT ×
VIN
VOUT
This formula has a maximum at VIN=2VOUT, where IRMS=IOUT/2. This simple worst‐ case condition is commonly used for
design because even significant deviations do not offer much relief.
Choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet size
or height requirements in the design.
For the input capacitor, a 10μFx2 low ESR ceramic capacitor is recommended. For the recommended capacitor, please
refer to table 3 for more detail. The selection of COUT is determined by the required ESR to minimize voltage ripple.
Moreover, the amount of bulk capacitance is also a key for COUT selection to ensure that the control loop is stable. Loop
stability can be checked by viewing the load transient response as described in a later section. The output ripple, ΔVOUT ,
is determined by:

1 
ΔVOUT ≤ ΔIL × ESR +

8fCOUT 

V1.0
5
March 2013
RS6508
The output ripple will be highest at the maximum input voltage since ΔIL increases with input voltage. Multiple capacitors
placed in parallel may be needed to meet the ESR and RMS current handling requirement. Dry tantalum, special
polymer, aluminum electrolytic and ceramic capacitors are all available in surface mount packages. Special polymer
capacitors offer very low ESR value. However, it provides lower capacitance density than other types. Although
Tantalum capacitors have the highest capacitance density, it is important to only use types that pass the surge test for
use in switching power supplies.
Aluminum electrolytic capacitors have significantly higher ESR. However, it can be used in cost-sensitive applications
for ripple current rating and long term reliability considerations. Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coefficient and audible piezoelectric effects. The high Q of ceramic
capacitors with trace inductance can also lead to significant ringing.
Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current,
high voltage rating and low ESR make them ideal for switching regulator applications. However, care must be taken
when these capacitors are used at input and output. When a ceramic capacitor is used at the input and the power is
supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, VIN. At best, this
ringing can couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long
wires can potentially cause a voltage spike at VIN large enough to damage the part.
OUTPUT RECTIFIER DIODE
The output rectifier diode supplies the current to the inductor when the high-side switch is off. To reduce losses due to
the diode forward voltage and recovery times, use a Schottky diode.
Choose a rectifier diode whose maximum reverse voltage rating is greater than the maximum input voltage, and whose
current rating is greater than the maximum load current.
CHECKING TRANSIENT RESPONSE
The regulator loop response can be checked by looking at the load transient response. Switching regulators take several
cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to
ΔILOAD(ESR) also begins to charge or discharge COUT generating a feedback error signal for the regulator to return VOUT to
its steady-state value. During this recovery time, VOUT can be monitored for overshoot or ringing that would indicate a
stability problem.
Component Supplier
MAGLAYERS
SUMIDA
TOKO
Component Supplier
MURATA
TDK
MURATA
TDK
VIN (Max.)
20V
V1.0
Part No.
B320
SK33
SS32
Series
MSCDRI-124-150M
CDRH104R
D104C
Table 1. Suggested Inductors for Application Circuit
Part No.
Capacitance (μF)
GRM31CR61E106K
10
C3225X5R1E106K
10
GRM32ER71C226M
22
C3225X5R1C226M
22
Table 2. Suggested Capacitors for CIN and COUT
Dimensions (mm)
12 x 12 x 5.0
10.1 x 10 x 3.0
10 x 10 x 4.3
Case Size
1206
1206
1210
1210
2A Load Current
Vendor
Diodes, Inc. (www.diodes.com)
Pan Jit International (www.panjit.com.tw)
General Semiconductor (www.gensemi.com)
Table 3. Schottky Rectifier Selection Guide
6
March 2013
RS6508
ABSOLUTE MAXIMUM RATINGS
Parameter
Supply voltage
SW pin voltage
Boot strap voltage
Feedback voltage
Enable/UVLO voltage
Comp voltage
SS voltage
Junction temperature
Operating temperature range
Storage temperature range
Lead temperature
Symbol
VIN
VSW
VBS
VFB
VEN
VCOMP
VSS
TJ
TOPR
TSTG
TLEAD
Range
-0.3 to +21
-0.3 to VIN +0.3
VSW -0.3 to VSW +6
-0.3 to +6
-0.3 to +6
-0.3 to +6
-0.3 to +6
150
-20 to +85
-40 to +150
260
Units
V
V
V
V
V
V
V
℃
℃
℃
℃
ELECTRICAL CHARACTERISTICS
(VIN=12V, TA=25℃, unless otherwise specified)
Parameter
Symbol
Input voltage
VIN
Feedback voltage
VFB
Upper switch on resistance
RDS(ON)1
Lower switch on resistance
RDS(ON)2
Upper switch leakage
ISw
Current limit (NOTE 1)
ILIM
Current sense transconductance output
GCS
current to comp pin voltage
Error amplifier voltage gain
AVEA
Error amplifier transconductance
GEA
Oscillator frequency
FS
Short circuit frequency
FOSC1
Soft-start pin equivalent output resistance
Maximum duty cycle
DMAX
Minimum on time
tON
EN shutdown threshold
Enable pull up current
EN UVLO threshold rising
EN UVLO threshold hysteresis
Supply current (Shutdown)
ISD
Supply current (Quiescent)
IQ
Thermal shutdown
TSD
Conditions
4.75V ≤ VIN ≤ 20V
VEN=0V, VSW=0V
-
Min.
4.75
0.898
-
Typ.
0.925
0.22
10
3.8
Max.
20
0.952
10
-
Unit
V
V
Ω
Ω
μA
A
-
-
1.95
-
A/V
VFB=0V
VFB=1.0V
ICC > 100μA
VEN=0V
VIN Rising
VIN ≤ 0.4V
VEN ≥ 3V
-
550
0.7
2.35
-
400
830
400
240
9
90
100
1.0
1.0
2.50
210
23
1.1
160
1150
1.3
2.65
36
1.3
-
V/V
μA/V
KHz
KHz
KΩ
%
ns
V
μA
V
mV
μA
mA
℃
Notes:
1. Slope compensation changes current limit above 40% duty cycle.
2. Stresses listed as the above "Absolute Maximum Ratings" may cause permanent damage to the device. These are for stress ratings. Functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied.
Exposure to absolute maximum rating conditions for extended periods may remain possibility to affect device reliability.
3. Devices are ESD sensitive. Handling precaution is recommended.
4. The device is not guaranteed to function outside its operating conditions.
5. θJA is measured in the natural convection at TA=25℃ on a high effective four layers thermal conductivity test board of JEDEC 51-7 thermal
measurement standard.
V1.0
7
March 2013
RS6508
PACKAGE INFORMATION
8-PIN, SOP
Notes:
1. Refer to JEDEC MS-012 AA.
2. All dimensions are in millimeter.
V1.0
8
March 2013
RS6508
IMPORTANT NOTICE
Princeton Technology Corporation (PTC) reserves the right to make corrections, modifications, enhancements,
improvements, and other changes to its products and to discontinue any product without notice at any time.
PTC cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a PTC product. No
circuit patent licenses are implied.
Princeton Technology Corp.
2F, 233-1, Baociao Road,
Sindian Dist., New Taipei City 23145, Taiwan
Tel: 886-2-66296288
Fax: 886-2-29174598
http://www.princeton.com.tw
V1.0
9
March 2013